The present invention relates to a method of producing oxidized white liquor in which the white liquor to be oxidized is formed from green liquor. More particularly the present invention relates to such a process in which dregs are separated from the green liquor and then are added to the white liquor to enhance the oxidation rate of the white liquor.
Wood pulp is processed into paper by digesting the wood pulp in a digester to which white liquor has been added. White liquor is an aqueous mixture of sodium sulfide and sodium hydroxide. Brown pulp stock, produced from the digestion of the wood pulp, is further delignified in a process known as oxygen delignification and then bleached in a series of stages which may use peroxide, ozone, or chlorine dioxide bleaching. White liquor is oxidized in order to deactivate the sodium sulfide which would otherwise react with the aforementioned bleaching agents. The degree of oxidation can be partial or complete and as such sulfides can be converted to thiosulfate or sulfate. The oxidized white liquor can then be used as a caustic source in the oxygen delignification process or the peroxide bleaching stages (peroxide bleaching would require complete oxidation or sodium sulfate) that are often provided prior to a chlorine dioxide bleaching stage.
A practical problem involved in the production of oxidized white liquor concerns the reaction time required to allow the oxidation of the sodium sulfide to go to the desired level of completion. Under ambient conditions, several hours are required to partly oxidize sodium sulfide and even longer time periods are required to produce sodium sulfate. Since large hydraulic retention times require a large capital investment, oxidized white liquor must be produced more rapidly than is possible under ambient conditions to make the use of oxidized white liquor economically feasible.
It has been found that the oxidation rate of white liquor can be accelerated by conducting the reactions at higher than ambient temperatures and pressures. In U.S. Pat. No. 5,500,085 white liquor is oxidized within a stirred reactor at a temperature range at between 180.degree. F. and 300.degree. F. and a pressure range of between 100 and 300 psig. As described in 37 Chemical Engineering Science, No. 2, pp. 327-336, Fast Reactions in Slurry Reactors: Catalyst Particle Size Smaller Than Film Thickness: Oxidation Of Aqueous Sodium Sulfide Solutions With Activated Carbon Particles As Catalyst At Elevated Temperatures, Sharma et al. (1982), an activated carbon catalyst added to aqueous sodium sulfide solutions will also reduce reaction times. The efficiency at which oxygen and white liquor are contacted with one another will also influence reaction time. In this regard, U.S. Pat. No. 5,439,556 illustrates a plug flow reactor employing structured packing that effects a reduction in reaction times by forming a descending film of the white liquor that contacts an ascending vapor containing the oxygen.
As will be discussed, the present invention provides method of oxidizing white liquor that is integrated into a pulping process to either partially or fully oxidize white liquor under practical reaction times.